1. Field of the Invention
The present invention relates to fiber optic sensors and particularly to a fiber optic Michelson interferometric sensor system which provides for the passive elimination of polarization fading and for the isolation of the optical source from feedback from the fiber optic interferometer sensor system.
2. Description of the Prior Art
The use of interferometric fiber optic sensors constructed using conventional low birefringence fiber presents two problems which bear on the effectiveness of those sensors in various sensing applications.
First, in such interferometric fiber optic sensors, random fluctuations in the state of polarization (SOP) of the interfering beams give rise to variations in the output fringe visibility, and consequently fading of the detected interferometric signal. This `polarization-induced fading` phenomenon is an important problem which affects the practical applicability of interferometric sensors in a number of application areas.
Second, typical Michelson interferometric fiber optic sensor systems apply light from an optical source or laser through a 3 dB optical coupler to the interferometric fiber optic sensor. The returned signal light returns to this optical coupler, with one-half of this returned signal light passing through one port of the optical coupler to an optical detector and the other half of this returned signal light passing through another port of the optical coupler back toward the laser. This returned signal light not only results in a power loss but can also be detrimental the laser, particularly when the laser is a diode laser. Thus, there is a need to isolate the optical source or laser from the signal light returned from the interferometric fiber optic sensor system to prevent possible damage to the laser and to also minimize power loss in the signal light before it is detected.